A variety of differently colored marking media are commonly used for drawing and coloring activities. Such marking media include, for example, differently colored markers, crayons, pens, pencils and paints. Oftentimes, it is desirable to change the color of a mark made by the aforementioned marking media after said mark has been made. For instance, one may want to draw a yellow flower over a previously colored green field. Changing the color of a mark after it has been made cannot readily be done with conventional marking media. In the past, to attempt to change the color of a mark after it has been made, one typically places a second mark of a desired color over a first mark of an undesired color. This approach is rarely successful when employing conventional marking media as the first and second marks tend to smear together into a dark, undesirably-colored mark. In addition, when this approach to changing colors is attempted using porous-tip markers, the tip of the second-used marker often becomes soiled and ruined when it is brought into contact with the mark left by the first marker.
Therefore, there has been a long-felt need for marking compositions well-suited for coloring and drawing applications which produce marks of a first color that can readily be changed into a second color.
One proposed solution to the above-described problem is described in German Patent Specification No. 2,724,820, published Jan. 1975, the disclosure of which is incorporated herein by reference. This patent specification describes a color changing marking composition system which comprises a pair of markers. The first marker contains a bleach-sensitive dye of a first color combined with a bleach-stable dye of a second color. The second marker contains a clear bleach solution. In use, a mark is applied using the first marker, the mark typically being a blend of the first and second colors. The second marker is then used to write over that portion of the mark wished to be changed in color. When the clear bleach solution of the second marker comes into contact with the bleach-sensitive dye present in the overwritten portion of the mark, the color contribution of the bleach-sensitive is eliminated therefrom, leaving only the color contribution of the bleach-stable dye.
There are certain drawbacks to the aforementioned marking system. First, since one of the pair of markers contains only the bleaching agent, that bleaching marker cannot be used to render a visible mark and may only be used in combination with the dye-containing marker. Consequently, once the dye-containing marker is used up, there is no use for the bleaching marker. Alternatively stated, once the bleaching marker is used up, the dye-containing marker can only be used for the color in which it initially marks. Second, as the bleach-containing marker is applied over an existing mark, its porous tip tends to pick up and get soiled by the dyes present in the existing mark. As a result of such soiling, the bleaching marker often taints the color of subsequent marks to which it is applied.
Another approach to this problem is described in U.S. Pat. No. 5,232,494, which issued Aug. 3, 1993, the disclosure of which is incorporated herein by reference. This patent also describes a color changing composition system which comprises at least two marking instruments. The first marking instrument contains a first ink composition comprising a dye whose coloring ability is destroyed in the presence of a bleach and/or a pH of about 10 or greater. The second marking instrument contains a second ink, the second ink comprising (1) from about 1% to about 20% by weight of bleach; (2) from about 0.1% to about 10% by weight of a base; and (3) from about 0.1% to about 15% by weight of a colorant which maintains its characteristic color in the presence of a bleach and/or a pH of about 10 or greater.
One drawback to the approach disclosed in the foregoing patent is that two separate markers, each marker containing a different marking composition, must be used for a color-changeable mark to be made. Consequently, if one of the markers is lost or breaks, a mark of only one color can be made.